1. Field of the Invention
The present invention relates to a an image forming apparatus and an image forming method which sequentially form developed images of colors corresponding to image signals of respective colors and transfer the formed developed images in a superimposed manner onto a sheet.
2. Description of the Related Art
Conventionally, as an electrophotographic color image forming apparatus, there has been an image forming apparatus in which image forming sections are provided for respective colors. Each image forming section of this image forming apparatus is comprised of a photosensitive drum, an exposure device, and a developing device. In each image forming section, laser light modulated in accordance with image data of the corresponding color is irradiated from the exposure device onto the photosensitive drum. As a result, a latent image of the corresponding color is formed on the photosensitive drum. Then, the latent image formed on the photosensitive drum is visualized as a toner image of the corresponding color by the developing device. That is, the image forming sections form toner images of respective colors.
The toner images formed by the respective image forming sections are primarily transferred in a superimposed manner onto a sheet supported on an intermediate transfer member or a transfer conveying member by a primary transfer section (multilayer-transfer). In a case where an intermediate transfer member is used, toner images of respective colors multilayer-transferred onto the intermediate transfer member are collectively transferred onto the sheet by a secondary transfer section (secondary transfer). As the intermediate transfer member, an endless belt (intermediate transfer belt) is used in many cases.
To increase the latitude of transfer from the photosensitive drum to the intermediate transfer belt (transfer efficiency) in the image forming apparatus using the intermediate transfer belt as above, it is necessary to set the primary transfer current value to the optimum value. It is, however, difficult to set the primary transfer current value to the optimum value, and there may be cases where the primary transfer current value is set to be a low value or a high value. If the primary current value is lower than the optimum value, a transfer failure may occur, and if the primary current value is higher than the optimum value, this may cause retransfer.
To address this problem, a technique for increasing the primary transfer latitude by driving a photosensitive drum and a transfer belt in such a manner as to produce a difference in peripheral speed between them. In the case of this technique, such a shearing force as to scrape out a toner image on the photosensitive drum is produced between the photosensitive drum and the intermediate transfer belt due to the difference in peripheral speed, and the shearing force is used for primary transfer of the toner image. With this technique, improvement and stabilization of the primary transfer latitude can be accomplished, and variations in image density and dropout of a lines or a character image (in particular, dropout in the middle section of a thin line of a secondary color) are prevented. In the case of the above technique, however, the coefficient of friction varies to change the rotational speed of the photosensitive drum depending on whether or not toner exists between the photosensitive drum and the intermediate transfer belt since frictional force is constantly produced between the photosensitive drum and the intermediate transfer belt due to the difference in peripheral speed. As a result, the position of laser light irradiated on the photosensitive drum may change, which could form stripe-shaped images.
The above phenomenon also occurs in an image forming apparatus of the type that is comprised of a plurality of developing devices arranged for one photosensitive drum, sequentially forms toner images of corresponding colors on the photosensitive drum, and superimposes the toner images on an intermediate transfer member. The above phenomenon also occurs in a case where a toner image is transferred onto a sheet conveyed from a photosensitive drum while being supported by a transfer material conveying member.
To cope with this, a method which prevents the occurrence of the above phenomenon has been proposed for an image forming apparatus which drives a photosensitive drum and an intermediate transfer member or a transfer material conveying member in such a manner as to produce a difference in peripheral speed between them (see e.g. Japanese Laid-Open Patent Publication (Kokai) Nos. 2004-118076 and 2004-151588). In this method, a dot-dispersed image (dot pattern) which is an additional image comprised of dispersed dot toner images is superimposed on a toner image which is a normal image formed in accordance with an image signal of the corresponding color. As a result, a high-quality image with no stripes (image stripes) can be obtained.
Also, in the case of the arrangement in which a photosensitive drum and an intermediate transfer member or a transfer material conveying member are driven in such a manner as to produce no difference in peripheral speed between them, an unexpected speed difference may be produced between them, which could cause a color shift. For such an arrangement, the above described method in which a dot-dispersed image (dot pattern) comprised of dispersed dot toner images is superimposed on a toner image which is a normal image is useful.
Incidentally, image forming apparatuses of an electrophotographic type using toners of four or more colors have been proposed in recent years. In such image forming apparatuses, light color toners such as light cyan toner and light magenta toner are used in addition to toners of four colors, i.e. cyan, magenta, yellow, and black (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. H11-52758). By using such light color toners, a high-quality image with decreased graininess can be obtained.
Such multicolor image forming apparatuses are required to form higher-quality images as compared with conventional four-color image forming apparatuses. However, if the conventional method in which a dot pattern which is an additional image comprised of dot dispersed toner images is superimposed on a toner image which is a normal image is used, the tint of an image may change; for example, areas with low density take on a yellow tinge. Thus, high-quality images desired by users may not be provided, and what is even worse, image quality may be degraded.